[2] Depreciation describes the devaluation of fixed capital through wear and tear associated with its use in productive activities.
NNP is a more accurate measure of total value of goods and services by a country.
As a rough estimate, GNP is very useful indicator of total production of a country.
But if we are interested to have an accurate and true measure of what a country is producing and what is available for uses, then GNP has a serious defect.
Nonetheless, the net national product has been the subject of research on its role as a dynamic welfare indicator[5] as well as a means of reconciling forward and backward views on capital wherein NNP(t) corresponds to the interest on accumulated capital.